1. Field of Invention
The present invention relates to display devices such as liquid crystal display devices which perform display by controlling a state of liquid crystal enclosed between substrates, or current-controlled display devices which perform display by making EL (electroluminescence) devices and LED (light-emitting diode) devices emit light by means of driving current. More particularly, the present invention relates to electrical connect techniques of conductive films constituting the electrodes and wiring in the display devices described above.
2. Description of Related Art
In display devices such as liquid crystal display devices and current-controlled display devices, display is controlled by using switching elements such as thin film transistors. Although transparent substrates are used as supports in such display devices, when general-purpose glass is used as the transparent substrate, the fabrication process of panels must be performed at low temperatures. Accordingly, recently, liquid crystal display devices and the like which use a polycrystalline silicon thin film transistor (hereinafter referred to as TFT) are actively developed.
In a polycrystalline silicon TFT fabricated by the low-temperature process, as shown in FIG. 8, island-like polycrystalline silicon films 2 are formed on a substrate 1, and a channel region 4, a source region 3, and a drain region 5 are formed on the island-like polycrystalline silicon films 2. A gate electrode 7 faces the channel region 4 with a gate oxide film 6 composed of a silicon oxide film therebetween. A TFT30 configured as described above is covered with a first interlevel insulating film 10 composed of a silicon oxide film. Contact holes 101 and 102 are opened through the first interlevel insulating film 10 at the positions corresponding to the source region 3 and the drain region 5. A source electrode 8 and a drain electrode 9 composed of aluminum or an aluminum alloy are electrically connected to the source region 3 and the drain region 5, respectively, through the contact holes 101 and 102.
Also, a second interlevel insulating film 11 composed of a silicon oxide film is formed on the front surface of the source electrode 8 and the drain electrode 9, and a contact hole 103 is made through the second interlevel insulating film 11 at the position corresponding to the drain electrode 9. Thus, a transparent display electrode 12 composed of an ITO film is electrically connected to the drain electrode 9 through the contact hole 103 in the second interlevel insulating film 11. Since the drain electrode 9 composed of an aluminum film and the transparent display electrode 12 composed of an ITO film are not electrically connected well to each other, by intervening a contact metal 17 composed of a molybdenum film (Mo) or the like between the drain electrode 9 and the transparent display electrode 12, the drain electrode 9 and the transparent display electrode 12 are in ohmic contact with each other.
However, in the configuration shown in FIG. 8, when the anneal process is performed after wiring layers such as the source electrode 8 and the drain electrode 9 are formed on the source region 3 and the drain region 5 composed of polycrystalline silicon, it has been found that contact failure occurs between the source electrode 8 and the source region 3 and between the drain electrode 9 and the drain region 5. That is, silicon in the polycrystalline silicon film diffuses into the aluminum film at the electrical connection (joining area) between the drain region 5 composed of the polycrystalline silicon film and the drain electrode 9 composed of the aluminum film, and thus, contact resistance increases. The contact resistance increases remarkably when the anneal process is performed at a temperature of 300xc2x0 C. or more after wiring layers such as the source electrode 8 and the drain electrode 9 are formed on the source region 3 and the drain region 5 composed of polycrystalline silicon. Such contact failure, which prevents a circuit from operating normally, results in display failure of pixels, which is a fatal defect in the case of a liquid crystal display device. However, in the known art, the problem of silicon diffusion into the aluminum alloy could not be solved completely.
Also, with respect to the method for bringing the drain electrode 9 composed of an aluminum film into ohmic contact with the transparent display electrode 12 composed of an ITO film, there has been only one method of intervening a metal film such as molybdenum, and freedom in design and process of liquid crystal display devices was limited.
In view of the problems described above, it is an object of the present invention to provide a new configuration of display devices such as liquid crystal display devices, in which electrodes and wiring can be connected with a low resistance.
In order to achieve the object described above, in accordance with the present invention, a display device having a first electrical connection between a polycrystalline silicon film and an aluminum film includes a first titanium nitride film provided between the polycrystalline silicon film and the aluminum film.
In the present description, an aluminum film means a film composed of aluminum alone and also means an aluminum alloy film including aluminum as a major constituent.
In accordance with the present invention, the first titanium nitride film intervenes between the polycrystalline silicon film and the aluminum film, and the first titanium nitride film functions as a diffusion-prevention film. That is, silicon does not diffuse from the polycrystalline silicon film into the aluminum film even if the anneal process is performed at a high temperature of 300xc2x0 C. or more after a wiring layer composed of the aluminum layer is formed on the polycrystalline silicon. Accordingly, the contact resistance at this portion can be maintained at a low level.
Also, in accordance with the present invention, preferably a second titanium nitride film is provided between the aluminum film and the ITO film at a second electrical connection between the aluminum film and a transparent display electrode composed of an ITO film through a contact hole in an interlevel insulating film. In accordance with the present invention, at the second electrical connection, since the transparent display electrode composed of an ITO film and the aluminum film are electrically connected to each other with the second titanium nitride film therebetween, the transparent display electrode (ITO film) and the aluminum film can be brought into ohmic contact with each other. Also, since the second titanium nitride film is formed on the surface of the aluminum film, even if wet-etching using a hydrofluoric acid-based etchant is employed when making a contact hole into the interlevel insulating film, what is exposed on the bottom of the contact hole upon completion of etching is the second titanium nitride film which is resistant to the hydrofluoric acid-based etchant, not the aluminum film. Thus, when the contact hole is made into the interlevel insulating film, etching does not affect the aluminum film. Also, when patterning is performed onto the ITO film, even if the interlevel insulating film has cracks or the like, what is touched by a bromine-based etchant for etching the ITO film is the second titanium nitride film which is resistant to the bromine-based etchant, not the aluminum film. Thus, disconnection or the like does not occur in the aluminum film. Therefore, in accordance with the present embodiment, reliable liquid crystal display devices can be fabricated.
Also, if the same titanium nitride film is provided between the polycrystalline silicon film and the aluminum film and between the aluminum film and the transparent display electrode (ITO film), deposition can be performed with the same target when the sputtering process is used for deposition. Therefore, as targets for sputtering deposition, two targets only are required, that is, a target for forming the aluminum film and a target for forming the titanium nitride (titanium) film.
In the configuration described above, preferably the first titanium nitride film and the second nitride film have the same film thickness and the same film characteristics. In such a configuration, stress caused by the first titanium nitride film to the aluminum film and stress caused by the second titanium nitride film to the aluminum film are substantially equal, and thus, the aluminum film is not forced unnecessarily. Therefore, defects such as cracks and separation do not occur in the first titanium nitride film, the aluminum film, the second titanium nitride film, and others. Also, since the first titanium nitride film for preventing diffusion and the second titanium nitride film, which is resistant to an etchant and which brings the ITO film and the aluminum film into ohmic contact, are formed under the same deposition conditions, the control of the conditions during deposition is facilitated.
In accordance with another embodiment of the present invention, a display device having a second electrical connection between an aluminum film and a transparent display electrode composed of an ITO film through a contact hole in an interlevel insulating film includes a second titanium nitride film provided between the aluminum film and the ITO film. In the present invention, a new material, i.e., a titanium nitride film, is presented in order to bring the ITO film and the aluminum film into ohmic contact with each other. The titanium nitride film, differing from a metal film, is resistant to the hydrofluoric acid-based etchant for making a contact hole into the interlevel insulating film, and is resistant to the bromine-based etchant for performing patterning to the ITO film, and thus, the aluminum film is protected. Therefore, freedom in design and process of liquid crystal display devices or the like will broaden.
In accordance with the present invention, preferably the first titanium nitride film and the second titanium nitride film are titanium nitride films having a hexagonal crystal structure. As mentioned above, if both titanium nitride films have a hexagonal crystal structure, since the first titanium nitride film for preventing diffusion and the second titanium nitride film, which is resistant to various etchants and capable of being in ohmic contact with the ITO, can be formed under the same deposition conditions, the control of the conditions during deposition is facilitated.
The present invention is applicable to a liquid crystal display device or the like in which the polycrystalline silicon film and the aluminum film are formed on either one of the substrates sandwiching liquid crystal.
When the display device of the present invention is fabricated, preferably after a titanium nitride film for constituting the first titanium nitride film is formed, an aluminum film is formed on the surface of the titanium nitride film, and then, patterning is performed simultaneously onto the aluminum film and the titanium nitride film to simplify the etching process.
When the display device of the present invention is fabricated, preferably after a titanium nitride film for constituting the first titanium nitride film is formed, an aluminum film is formed on the surface of the titanium nitride film, and a titanium nitride film for constituting the second titanium nitride film is formed on the surface of the aluminum film, and then, patterning is performed simultaneously onto the aluminum film and the titanium nitride films formed on the upper and lower surfaces of the aluminum film to simplify the etching process.
When the display device of the present invention is fabricated, preferably after the aluminum film is formed, a titanium nitride film for constituting the second titanium nitride film is formed, and then patterning is performed simultaneously onto the aluminum film and the titanium nitride film to simplify the etching process.